You can remove the binding, unstitch the quilting around a couple of inches. Add more to the top and back. Then for the batting, you would butt a piece of batting up to the existing batting and use some of that iron on tape to join them. That keeps the batting flat. Smooth the top and backing over the batting and quilt. Bind as usual.
One time I saw a video or tutorial on this, but I cannot locate it now. Maybe someone else remembers where it is.